Validator-Lite

Validator-Lite is a simple but rigorously-correct validator for HTML,
XHTML, and other DTD-based XML languages. Its basic purpose is to offer
a bare-bones GUI for users who find "raw" OpenSP tools such as nsgmls
difficult or confusing. It is a lighter weight tool than the webserver-
based validators, and by installing everything locally it runs faster,
and works when you're offline.